(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, to a reflective LCD device or transflective LCD device having a reflective film which reflects light incident from outside the LCD device toward an observer.
(b) Description of the Related Art
An LCD device includes an LCD panel, which acts as light switches for controlling the transmission of light, pixel by pixel, for display of images thereon. The LCD panel includes a liquid crystal (LC) layer, a pair of substrates sandwiching therebetween the LC layer, and a pair of polarizing films, each disposed in the vicinity of a corresponding substrates. The LC layer controls the polarization of light and the polarizing films control the transmission of light therethrough based on the polarized direction of the light.
The LCD devices are generally categorized in three types: transmissive type, reflective type and transflective type. The transmissive LCD device uses a light source disposed at the rear side of the LCD panel for displaying images. The reflective LCD device includes a reflective film disposed at the rear side of the LC layer for reflecting the light incident from the front side of the LC layer to pass again through the LC layer for displaying images. The transflective LCD device includes, in each of an array of pixels, a transmissive part having a function similar to the function of the transmissive LCD device and a reflective part having a function similar to the function of the reflective LCD device.
The reflective LCD device, as well as the transflective LCD device, has an advantage of lower power dissipation compared to the transmissive LCD device, and thus is generally used in a portable terminal such as cellular phone and personal digital assistant. The term “reflective LCD device” as used hereinafter, generally includes the transflective LCD device for the sake of convenience, if not recited otherwise.
A recent reflective LCD device includes, as a basic structure thereof, a twisted nematic (TN) LC layer, an array of switching elements for driving the LC layer pixel by pixel, and a reflective film disposed outside or inside the LCD panel. The reflective LCD device is generally driven in an active-matrix-drive scheme, wherein thin film transistors (TFTs) or metal-insulator-metal (MIM) diodes are used as the switching elements for achieving a higher resolution and a higher image quality. A bottom electrode, i.e., pixel electrode connected to the switching element in each pixel is generally used as a part of the reflective film.
Patent Publication JP-B-2825713 describes a reflective LCD device having a reflective film used as pixel electrodes. The surface of the reflective film has a convex and concave pattern corresponding to each pixel and including a plurality of dots or convex portions, which are disposed periodically and protrude from the concave portions. It is recited in the publication that each dot reflects the light incident from a variety of directions toward the direction normal to the LCD panel to thereby improve the brightness and thus the image quality of the LCD panel.
The reflective film is manufactured in the process as described below. A photosensitive organic film is formed on an underlying film, exposed to light through a mask pattern and etched for patterning by using a photolithographic and etching step. The resultant photosensitive organic film has discrete dots, i.e, dots isolated from one another. An interlayer dielectric film and a reflective film are formed on the discrete dots and the underlying film, whereby the surface of the reflective film has smooth, convex portions corresponding to the locations of the dots.
It is known that the light reflected by the reflective film as described above assumes a rainbow color due to the regular pattern of the reflective film which causes interference of light. The rainbow color causes a problem of degradation in the image quality of the LCD device.
Patent Publication JP-B-3012596 describes a solution for the problem of the rainbow color in the reflected light in the LCD device, wherein the dots on the surface of the reflective film are randomly disposed in each pixel to thereby suppress the interference of light. In this publication, the pattern of the dots is common to all the pixels for reducing the amount of work for forming the mask pattern.
Although the problem of the rainbow color is suppressed by the structure described in Patent Publication JP-B-3012596, the solution is not sufficient in view of the image quality required of the recent LCD device. For further suppressing the above problem, it may be considered to use a bead adhesive layer between the front substrate and the polarizing film, the bead adhesive layer being such that a large number of beads are dispersed in the adhesive having a refractive index different from the refractive index of the beads. However, the bead adhesive layer reflects the light as a noise light to thereby degrade the contrast ratio of the LCD device by raising the luminance in display of a black color image.
Patent Publication JP-B-3066192 proposes to suppress the rainbow color in the reflected light by forming dots having different heights for suppressing the rainbow color. However, this configuration is also insufficient for suppressing the rainbow color, especially in the transflective LCD device having a smaller area of the reflective film in each pixel because smaller area intensifies the periodicity of the dots to thereby intensify the rainbow color.
It may be considered to suppress the rainbow color by randomizing the locations of dots in an area including a plurality of pixel areas. However, randomizing of the locations of dots in such a larger area generally requires a larger amount of work and thus is unpractical. In addition, the relation is not known in the art between the degree of randomization of the dots and achievement in the practical degree of suppression of the rainbow color.